This invention is related to the control of current through a motor using for example, a Pulse Width Modulation method of control. This type of regulation principle is similiar to a DC to DC converter where the supply voltage can be reduced with almost no energy loss. The converter consists of a periodically conductive switch and a free wheeling diode. The motor inductance is used for energy storage so that no additional storage inductance is required. When the switch is closed, the motor inductance is charged with current. When the switch is opened, the current is discharged via the free wheeling diode back through the motor. Therefore, during operation, a continuous motor current flows, the ripple of which is very low.
In the prior art there are examples of circuits which are designed to bypass this current around the motor so that the current can flow in the load even though the switch is open. In general, this turns out to be something like a diode which is connected in parallel across the motor in such a way as to oppose the supply voltage. This diode is then referred to as a Free Wheeling Diode. The only trouble is that for some types of circuits which use a switch means with an integral reverse diode, if the power supply is connected backwards a short circuit will exist through the switch and the free wheeling diode.